The 8 experiments proposed here investigate the nature of skill development in the production and perception of English speech sounds by comparing subject groups expected to differ in overall level of skill: child and adult native English speakers, and adults learning English as a foreign language. The major hypothesis to be tested is that phonetic representations become increasingly adaptive due to experience. This should be revealed in a variety of ways. Perceptual analyses should reveal that experienced talkers compensate better than less experienced talkers for the presence of an experimental dental prosthesis, and adapt more successfully for a bite-block rendering the jaw immobile during speech. Adults should adapt/compensate better for alterations of the normal configuration of the vocal tract when producing native-language vowels than unfamiliar, foreign-language vowels. The greater lip closing velocity of English /p/ than /b/ should be preserved better in bite block speech and speech produced at fast rates. Experienced talkers should also be more adept at producing /b/ with a cavity expanding gesture (useful for sustaining voicing) in a variety of circumstances. We predict that the domain of coarticulation will increase with experience. Experienced talkers should lower the velum (a gesture needed to produced, e.g., the /m/ in "Tom") sooner than less experienced talkers. In vowel producing we expect to observe an earlier, and perhaps greater, influence of following vowels on tongue shape. Experience with the sounds of a specific dialect should lead to increased adaptiveness in perception if humans continue learning the auditory properties of sounds beyond the time they can simply recognize spoken messages. We predict that adults will recognize a larger variety of naturally produced /t/ variants as /t/, and be better able to detect those variant falling outside the "prototypical" range of their native language. Adults of different language backgrounds should prefer different /t/ variants, yet they should all reveal a narrower range of preferences than children.